The present description relates to a spark ignited internal combustion engine with a fuel injector that injects fuel directly into a combustion chamber, or a so called direct injection spark ignition engine.
In recent years, to improve fuel economy and emission performance of spark ignited engines such as gasoline engines, there has been development of lean-burn engines. The lean-burn engine combusts a mixture of air and fuel with an overall air fuel ratio leaner than the stoichiometry of the fuel. The lean mixture contains greater amount of air than that of the stoichiometric air fuel mixture. Therefore, intake air is less throttled compared to a stroichiometric operation, and a pumping loss caused by the intake air throttling can be reduced to improve the fuel economy.
Generally, air fuel mixture is harder to ignite as the air fuel ratio is leaner. A conventional port injection engine injects fuel into an intake port, and the fuel is inducted with the intake air into the combustion chamber in an intake stroke of an engine cycle. When the mixture is ignited, it is homogeneous mixture in the combustion chamber, and the air fuel ratio around the spark plug is substantially same as that in anywhere else in the combustion chamber. Therefore, the air fuel ratio can not be set lean enough to achieve such fuel economy benefit.
On the other hand, the direct injection spark ignited engine uses a high pressure injector, which injects fuel directly into its combustion chamber. The direct fuel injection can create stratified mixture of air and fuel within the combustion chamber particularly when the fuel is injected in a compression stroke of an engine cycle. The stratified air fuel mixture contains both fuel rich mixture, which has an air fuel ratio of the stoichiometry or richer than that, and fuel lean mixture. If the rich mixture is in the proximity of the spark plug when it makes a spark, the rich mixture can be ignited and start combustion. Then, flame propagates from the ignited rich mixture to the lean mixture and completes the combustion of the whole mixture. Therefore, an overall air fuel ratio of the stratified air fuel mixture can be set leaner than that of the homogeneous air fuel mixture.
As described above, to create the stratified air fuel mixture with sustainable ignitability, the rich mixture needs to be in the proximity of the spark plug, and the lean mixture needs to surround the rich mixture. A method to create the stratified air fuel mixture is presented in U.S. Pat. No. 6,892,695. The method uses a configuration of intake port and makes a swirl flow of air in the cylinder around the cylinder axis in an intake stroke. Then, it uses a recess formed on a top surface of a piston and maintains the swirl flow. In a later stage of a compression stroke, it injects fuel from a side mounted injector toward a center mounted ignition plug, and then the injected fuel impinges the swirl flow. Consequently, this method may create the stratified air fuel mixture with rich mixture in the proximity of the spark plug when the mixture is ignited at timing around the top dead center of the compression stroke.
Another method to create the stratified air fuel mixture is presented in Japanese Patent Application Publication JP2005-256630A. The method uses a side mounted multi-hole injector having six injection holes. From two of the injection holes, fuel is injected in the proximity of a center mounted spark plug. Fuel from the rest of the injection holes impinges a curved wall of a recess, which is formed on a top surface of the piston, when the injection timing is after the middle of compression stroke, and then the fuel is dispersed. Therefore, this method can create the properly stratified mixture as well.
The method presented in the '695 patent has the recess on the piston to maintain the swirl airflow, and the method presented in the JP2005-256630A has the recess on the piston to disperse the injected fuel to create the lean mixture. The above methods may improve the creation of the stratified air fuel mixture. However, considering that fuel spray is less likely to deflect its way and most likely to adhere to a surface when it impinges the surface at a right angle because the movement energy is fully converted to energy of adhesion then, the inventors herein have recognized that the injected fuel may adhere to the recess surface presented in the prior arts. When some fuel adheres to the wall, particularly if it is injected at a later stage of compression stroke, that fuel may not be evaporated enough because of insufficient time for the evaporation by the time of ignition. Eventually, it may create a wet spot on the surface. Air fuel mixture in the proximity of the wet spot is over-rich of fuel, and it may cause smoke in the exhaust due to lack of enough oxygen. Or, a part of the mixture may be self-ignited and cause knocking. Therefore, the inventors herein have realized need to improve the piston recess configuration to create properly stratified air fuel mixture within the combustion chamber.